The process of this invention pertains to the preparation of water-dilutable coating compositions.
Increasing concern about environmental and health hazards has given impetus to the search for processes to prepare acceptable coatings without the use of organic solvents. A practical, useful invention not only requires that the coating compositions be in an aqueous medium, but the resulting coatings after curing must be essentially equivalent in properties to coatings prepared from present organic solvent systems.
The process of this invention involves the preparation of water-dilutable coating compositions. The "water-dilutable" as used herein indicates the formation of compositions which contain organic solids that are colloidally dispersed in water with no particles being visible when the composition is viewed through an optical microscope. Particle sizes present in the compositions do not exceed 0.1 micron. Moreover, this water-dilutability of the compositions of this invention is accomplished without the aid of water-miscible organic cosolvents.
The water-dilutable compositions of the process of this invention are distinguished from water-soluble or wateremulsifiable compositions, but the terms "water-borne," "water-reducible" and "water-dispersable" may be used interchangeably with "water-dilutable" within the definitions of said process.
The advantages of such a water system are a reduction in fire hazards since flammable organic solvents are replaced by water, the lack of odors and noxious and toxic fumes to be controlled during the manufacture and subsequent use of the coating composition, the lack of need for an expensive solvent recovery system which would be mandated by both economic and ecological reasons, and the fact that high solids compositions can be used expeditiously without the coating compositions becoming viscous and difficult to apply evenly thus allowing faster line speeds in coatings operations.
Increasing concern about environmental and health hazards, as exemplified by the California Rule 66 ban on the release of organic vapors into the atmosphere and in recent OSHA regulations on banning undue exposure of workers to potentially toxic organic vapors often encountered in conventional coating operations, points to the need for replacing organic solvents in coating compositions with water dilutable systems. The compositions of this invention provide a practical means of preparing coatings based on epoxy and aminoplast curing agents, which are known to give good coating performance, in a new and desirable process using aqueous systems thereby totally circumventing the hazards to personnel and the environment inherent with the conventional processes using organic solvents.
The coatings prepared from the compositions of this invention are essentially comparable in physical properties, appearance, durability, clarity, toughness, adhesion, strength, impact resistance and the like to coatings prepared from conventional coating compositions in organic solvents. Surprisingly these cured coatings are also essentially as insensitive to water as coatings prepared from conventional organic solvent systems.
U.S. Pat. No. 2,819,222 discloses the process for breaking petroleum emulsions employing certain oxyalkylation products derived in turn from nitrogen-containing compounds and polyepoxides. The polyepoxide starting materials covered by this patent are limited to monomeric diepoxides from polyhydric phenols or to low molal members (not over the tetramer) of the series and the products may be free of hydroxy groups. This is stated as important in this patent because U.S. Pat. No. 2,819,222 is directed toward products which are not resins and have certain solubility characteristics not inherent in resins. In addition, the oxyalkylation reaction is taught as being carried out not only with ethylene oxide which would give water-dispersible materials, but also with propylene oxide and butylene oxide which would not give the water-dilutability required of the instant products.
The instant products are resins which must be converted into tough, durable coatings after appropriate heatcuring of the instant compositions, quite the opposite of the teachings of U.S. Pat. No. 2,819,222, and for purposes not contemplated or envisioned by said patent teachings.
U.S. Pat. 2,819,222 limits the starting epoxide compounds to materials having values of n' of 0 to 3, preferably where n' is 0. These materials correspond to epoxy resins of molecular weight from about 340 to about 1,192, preferably about 340. This limitation is required to prepare soluble materials having the claimed demulsifier activity. U.S. Pat. No. 2,819,222 clearly implies that when n' is over 3, solubility (or water dilutability) no longer is attained.
It is known in the art that epoxy resins with molecular weights over 1,200, where n' is over 3, can be used in making resinous products. U.S. Pat. No. 2,819,222 also teaches this, but clearly does not teach or suggest that such higher weight epoxy resins can be modified as in the instant process to yield water-dilutable compositions which are useful in preparing coatings having acceptable physical properties.
The products of U.S. Pat. No. 2,819,222 are indeed water-dilutable and cured coatings can be prepared therefrom. However, the properties of said coatings are unacceptable weight epoxy resins (n' is 0 to 3) lead to films or coatings having unacceptable tackiness. This is a serious problem which must be avoided since it is customary to stack metal sheets coated with epoxy resin on top of one another. The stacked sheets are required to support appreciable weight without sticking together. The products of U.S. Pat. No. 2,819,222 fail to pass this critical requirement. "p When epoxy resins of higher molecular weight (1,500 to 5,000, where n is 4 to 17) are used in the instant process, tack-free coatings are obtained. Thus the instant invention is clearly differentiated from U.S. Pat. No. 2,819,222 both by the teachings of said patent and by the instant working Examples.
The instant compositions of this invention prefer that the poly(1,2-epoxide) starting material have significantly higher molecular weights, at least higher than the tetramer of the diepoxide monomer used so that ultimate coatings prepared from these water-reducible, heat-curing compositions will have practicably useful properties.
In the U.S. Pat. No. 2,731,444 the reaction of epoxy resins with polyhydric alcohols is taught to give compositions suitable as crosslinking reactants. There are disclosures of the reactions of epoxy resins with less than one equivalent of hydroxyl per epoxy group, with equivalent amounts of hydroxyl to epoxy group, but with incomplete reaction thereof, and of excess hydroxyl group per epoxy group. However, this patent does not teach or suggest the process of the instant invention.
While U.S. Pat. No. 2,731,444 teaches the reaction products of epoxy resins and polyhydric alcohols, including polyethylene glycol, can be further reacted with various polyfunctional crosslinking reactants through epoxy or hydroxy groups thereon and that the reaction products of epoxy resins with less than equivalent amounts of polyhydric alcohols are valuable coating compositions, it does not teach or suggest the process of this invention for the preparation of water-dilutable, heat-curable coating compositions nor does it lay down the critical limits required to achieve the instant process. In fact U.S. Pat. No. 2,731,444 teaches away from the instant process by broadly disclosing the reaction of epoxy resins and polyhydric alcohols without suggesting that water-dilutability is possible, let alone a desirable objective.
U.S. Pat. No. 2,951,778 teaches the use of the reaction product of poly(1,2-epoxide) with monomeric ethylene glycol at mole ratios of 2:1 to 1:2 to form a flexibilizer useful in epoxy resin formulations. Such as material would be insufficiently soluble in water to have any value in the water-dilutable process of this invention.
U.S. Pat. No. 2,947,717 teaches the use of a dihydric alcohol of molecular weight less than 600, including polyethylene glycol, in combination with a poly(1,2-epoxide) and a polycarboxylic acid anhydride for the preparation of crosslinked infusible resinous products made more fluid and hence easier to use before curing by the presence of dihydric alcohol. The various ingredients are mixed in the absence of solvent (organic or water) and heated until homogeneous at temperatures not over 80.degree. C. Heating at 80.degree.-200.degree. C affects curing and crosslinking of this system as all three components interact with one another. This system would not be water-dilutable, in fact the presence of water would render it inoperable due to premature hydrolysis of the anhydride before curing.
The reaction of polyethylene glycol with an aromatic diepoxide is also taught in U.S. Patent No. 3,563,943 for the preparation of oxyethylene-containing non-ionic compounds useful as emulsifiers for urethane latices. This patent teaches that the polyethylene glycol must have a molecular weight of from about 5,000 to about 10,000 and be reacted with an aromatic diepoxide in a mole ratio of polyethylene glycol to aromatic diepoxide of at least 2:1. The non-ionic emulsifiers resulting have a molecular weight of from 12,000-24,000. The instant process is not suggested by this reference.